1. Field of the Invention
The present invention relates to a technique for removing a silicon natural oxidation film which is formed on a silicon substrate and realizing an ohmic contact during manufacturing of a semiconductor device which is formed on the silicon substrate.
2. Description of Related Art
Normally, a semiconductor device comprising a silicon substrate includes a polysilicon film for creating electric connection to a diffusion layer which is formed in the upper part of the silicon substrate.
FIG. 6 is a conceptual view showing a structure of a conventional polysilicon film manufacturing apparatus. Silicon substrates 3 are supported upright on a quartz boat 4 within a quartz chamber 1, and the air inside the quartz chamber 1 is exhausted by a dry pump 7 through a vacuum exhaustion pipe 5. The pressure inside the quartz chamber 1 is adjusted by means of a pressure control valve 9 which is attached to the vacuum exhaustion pipe 5 while heating the quartz chamber 1 by a heater 2, during which an N.sub.2 gas is introduced into the quartz chamber 1 through an air operate valve 8a as a carrier gas to carry an SiH.sub.4 gas into the quartz chamber 1 through the air operate valve 8b. At this stage, a main valve 6 is open. Low pressure chemical vapor deposition realized by this makes it possible to form a polysilicon film.
On surfaces of diffusion layers of the silicon substrates 3, silicon natural oxidation films are formed due to oxygen which is contained in air. If polysilicon films are formed on the diffusion layers with the silicon natural oxidation films left as they are, the silicon natural oxidation films make it difficult to realize an excellent ohmic contact between the diffusion layers and the polysilicon films. To avoid this inconvenience, a mixed solution of pure water and hydrofluoric acid is conventionally used to remove the silicon natural oxidation films before creating the polysilicon films.
However, when such a mixed solution as above is used, particles contained in the mixed solution adhere to the surfaces of the silicon substrates 3. Further, the silicon natural oxidation films grow once again during the storage period of the silicon substrates 3 between removal of the silicon natural oxidation films by means of the mixed solution and deposition of the polysilicon films.